It is often desirable to use personal listening devices when listening to music and other audio material, or when participating in a telephone call, in order to not disturb others that are nearby. When a compact profile is desired, users often elect to use in-ear earphones or headphones, sometimes referred to as earbuds. To provide a form of passive barrier against ambient noise, earphones are often designed to form some level of acoustic seal with the ear of the wearer. In the case of earbuds, silicone or foam tips of different sizes can be used to improve the fit within the ear and also improve passive noise isolation.
With certain types of earphones, such as loose fitting earbuds, as well telephone handsets, there is significant acoustic leakage between the atmosphere or ambient environment and the user's ear canal, past the external surfaces of the earphone or handset housing and into the ear. This acoustic leakage could be due to the loose fitting nature of the earbud housing, which promotes comfort for the user. However, the additional acoustic leakage does not allow for enough passive attenuation of the ambient noise at the user's eardrum. The resulting poor passive acoustic attenuation can lead to lower quality user experience of the desired user audio content, either due to low signal-to-noise ratio or speech intelligibility especially in environments with high ambient or background noise levels. In such a case, an ANC mechanism may be effective to reduce the background noise and thereby improve the user's experience.
ANC is a technique that aims to “cancel” unwanted noise, by introducing an additional, electronically controlled sound field referred to as anti-noise. The anti-noise is electronically designed so as to have the proper pressure amplitude and phase that destructively interferes with the unwanted noise or disturbance. An error sensor (typically an acoustic error microphone) is provided in the earphone housing to detect the so-called residual or error noise. The output of the error microphone is used by a control system to adjust how the anti-noise is produced, so as to reduce the ambient noise that is being heard by the wearer of the earphone. In some cases, there is also a reference microphone that is positioned some distance away from the error microphone, and whose signal is used by certain ANC algorithms. The ANC controller operates while the user is, for example, listening to a digital music file that is stored in a local audio source device, or while the user is conducting a conversation with a far-end user of a communications network in an audio or video phone call, or during another audio application that may be running in the audio source device. The ANC controller implements digital signal processing operations upon the microphone signals so as to produce an anti-noise signal, where the anti-noise signal is then converted into sound by the speaker driver system.